On the deepest dive ever made by a human inside a submarine, a United States explorer saw something he could have found in the gutter of nearly any street in the world: rubbish.

Key points:

The dive went 16 metres lower than the previous deepest sea trench descent

The expedition believes it found manmade objects in the trench, thought to be plastic

At least three new species of sea creatures were identified during the mission

Victor Vescovo, a retired naval officer and Texas-based investor, said he made the unsettling discovery as he descended nearly 10,928 metres to a point in the Pacific Ocean's Mariana Trench that is the deepest place on Earth.

Mr Vescovo also saw angular metal or plastic objects, one with writing on it.

"It was very disappointing to see obvious human contamination of the deepest point in the ocean," he said.

New species discovered

In the past three weeks, the expedition has made four dives in the Mariana Trench in Mr Vescovo's submarine, DSV Limiting Factor, collecting biological and rock samples.

According to Five Deeps Expedition, their scientific team identified at least three new species of marine animal during the dive series, which also went to the bottom of the Indian, Southern and Atlantic oceans.

A type of long-appendaged amphipod was discovered at the bottom of the Challenger Deep, in the Mariana Trench.

In the Java Trench, the deepest point of the Indian Ocean, researchers identified a gelatinous animal — thought to be a stalked ascidean, otherwise known as a sea squirt — which they said does not resemble anything seen before.

"It is not often we see something that is so extraordinary that it leaves us speechless," said chief expedition scientist Dr Alan Jamieson.

"Amongst many other rare and unique observations, the stalked ascidean was a really significant moment."

Previous records broken

Mr Vescovo is the first person to make multiple dives to the depths of the Mariana Trench, where on one occasion he spent four hours on the bottom.

His latest dive went 16 metres lower than the previous deepest descent in the trench in 1960.

"It is almost indescribable how excited all of us are about achieving what we just did," Mr Vescovo said after arriving in Guam after the completion of the dives.

"This submarine and its mother ship, along with its extraordinarily talented expedition team, took marine technology to an unprecedented new level by diving — rapidly and repeatedly — into the deepest, harshest area of the ocean."

Titanic and Avatar director James Cameron was the last to visit in 2012 in his submarine, reaching a depth of 10,908 metres.

The first-ever expedition to Challenger Deep was made by the United States Navy in 1960.